I couldn't feel any difference on the Grand Totals as you are projecting same Sales data for future sales.
Also, formula for future year sales depicts as previous year when you say -12.
Wasn't it supposed ot be like this: YEAR([Order Date]) >= YEAR(Order Date)-1?.
The future sales do have differences, primarily in months where there is no value for sales (see below, the future sales number pulls from same month prior year). And looking at the monthly totals I need to look at same month prior year, so 12 columns to the left. Granted these are small numbers but my real data has larger differences. It's the grand total for future sales that's not working because of the table calculation I assume.
I had not, but just tried to follow the steps and it doesn't seem to work. Not sure what I'm doing wrong.
Is the attached fine? Totals for 2014 are slighly higher than TOTAL(SUM([Sales]))
For best explanation please refer to this awesome video: Customising Grand Totals - YouTube